M O L L U S C A. 



599 



Mollusc*, internal structure, not having been previously deter- 

 *""V^* > mined, these characters were obviously artificial, and 

 the unnatural combinations which resulted from their 

 employment, displayed themselves in every genus of 

 any extent. In the same genus animals were to be 

 found which respire in air, associated with those which 

 perform the same function by means of gills in water ; 

 or animals whose gills are like leaves placed external- 

 ly, with tbose having their gills in an internal cavity. 

 Such incongruous combinations chiefly prevail among 

 the vermes testacea, where the shell is exclusively em- 

 ployed in the determination of genera and species. 

 There is another imperfection in this system, arising 

 from the separation of the naked and testaceous inol- 

 lusca into distinct orders. There is no such line of 

 distinction observed by nature ; and very considerable 

 difficulties occur in its practical application as an arti- 

 ficial arrangement. Not a few animals included in the 

 testacea, as the Bullaaperta, have the shell so concealed 

 under the kin, that it can only be rendered visible by 

 the separation of the softer puts ; and several animals, 

 included among the naked mollusca, are covered, in 

 particular places, by a corneous, or shelly plate, as the 

 Aplysia;. In consequence of these circumstances, ge- 

 nera which are nearly related are placed in different 

 orders, while those which posses* few common pro- 

 perties are grouped together. 



Independent of these objections, which have been 

 made against the Linnzan classification of molluscous 

 animals, it is still pertinaciously adhered to by many 

 British naturalists. This influence of the Uolum thca- 

 Iri is the more to be regretted, as it has retarded the 

 progress of science, limited the field of observation, 

 and prevented us from availing ourselves of the im- 

 provements which have resulted from the labours of 

 our continental neighbours. 



For many years the influence of the artificial me- 

 thod of Linn.Tus, in botany, had been successfully as- 

 smiled in France ; and the followers of the natural me- 

 thod at length became so numerous as to secure it a 

 favourable reception. Various efforts were likewise 

 oade to promote its introduction into the science of 

 zoology. This was at length triumphantly effected, 

 in regard to molluscous animals, by M. G. Cuvier. 

 Other enquirers, it is true, largely contributed to the ac- 

 compliglunent of this desirable end : three of whom, dis- 

 tinguished by their patient industry, deserve to be parti- 

 cularly enumerated, Adamson, Muller, and I'oli. The 

 former paved the way to the modem arrangements, in 

 hi* Hutoire Xalurelie du Senegal. Muller explored the 

 molluscous animals of Denmark, and published the 

 result of his observations in the Zootogia Danica, and 

 his Vermium Terretlrium ct FliroiatUium Hutoria. 1'oli, 

 directing his attention to the structure of the animals 

 inhabiting the multivalve and bivalve shells, publish- 

 ed his laborious observations in his Tettacta vtriiumie 

 Sicilia eoruntque Hutoria el Analome. Many other 

 authors might be enumerated, who have likewise con- 

 tributed to extend our knowledge of the structure of 

 molluscous animals, as Lister, Monro, and Home ; but 

 as we shall have occasion afterwards to advert to their 

 labours, it is unnecessary to enter into further details. 

 M. CuTir. The first efforts of Cuvier to illustrate this depart- 

 ment of zoology, though necessarily imperfect, excit- 

 ed the attention of naturalists to the subject, and were 

 the prelude to those mighty achievements more recent- 

 ly made known to the world in his various papers in- 

 serted in the Annalet du Muteum, and repubiished un- 

 der the title Mrmoiret pourtfrrir a f Hutoire el a C Ana- 

 tomie det Molliuqvct,inone volume quarto, Paris, 1816. 



These papers may be considered as models of "minute Mollutcr 

 and accurate research, perspicuous description, and * ~~Y~" 

 candid criticism, and merit the careful and frequent 

 perusal of the student of molluscous animals. In the 

 second volume of a more recent work by the same au- 

 thor, entitled, Le Rt-gne Animal destrihue d'apres ton 

 Organization, Paris, 1817, he has given a synop- 

 tical view of the subject, which we intend to make 

 the basis of the following article. 



As a preliminary step to an examination of the di- 

 visions of molluscous animals, we shall take a very ge- 

 neral view of their structure and functions, reserving 

 the more minute details to be given under the differ- 

 ent groups into which they have been arranged. In- 

 deed, the several organs, as they appear in the differ- 

 ent tribes, are so variously modified, that few common 

 properties can be enumerated. In this section, how. 

 ever, we shall be able to mark the course to be observ. 

 ed in the illustration of the subordinate divisions, en- 

 deavouring, at the same time, to avoid saying any 

 thing which will again require to be repeated. 



1. General Form. The molluscous animals exhibit General 

 very remarkable differences in their form, and the num- form. 

 her and positionof their external members. Neither head 



nor foot can be observed in some, the principal organs 

 being enclosed in a bag pierced with apertures for the 

 entrance of the food and egress of the excrementitious 

 matter. In some, whose exterior is still remarkably 

 simple, cuticular elongations, termed tentacula, sur- 

 round the mouth, and a foot, or instrument of mo- 

 tion, may likewise be perceived. This last organ is in 

 some free at one extremity, in others attached to the 

 body throughout its whole length. In many species 

 there is a head, not, however, analogous to that mem- 

 ber in the vertebral animals, and containing the brain 

 and organs of the senses, but distinguished as the ante- 

 rior extremity of the body, separated from the back by 

 a slight groove, and containing the mouth and tentacula. 

 In many of the animals of this division, the differ- 

 ent members of the body are in pairs, and are arrang- 

 ed, in reference to a mesial plane, into right and left. 

 In some, part of whose organs respect a mesial plane, 

 other parts are single, or in unequal numbers. In 

 other species, the organs, which are not in pairs, are 

 arranged round a central axis, and give to the external 

 form a radiated appearance. But these characters are 

 exceedingly variable and uncertain, as marking the li- 

 mits of particular tribes ; since, in different parts of 

 the same animal, modifications of these forms may be 

 readily distinguished. 



2. Cutaneous System. The skin of molluscous ani. Cutaneoui 

 mals is more simple in its structure than the same or- system. 

 gan in the vertebral animals. The cuticle is here very 

 distinct ; and, as in other classes, it is thick and coarse 



where much exposed, but thin and delicate in its tex- 

 ture, where it lines the internal cavities. A mucuut 

 web may be detected in the cuttle fish and slug, but 

 of great tenuity. The corium is destitute of a villous 

 surface ; and on its central aspect it is so intimately 

 united to cellular substance, that its fibrous structure 

 can scarcely be distinguished. The muscular web, 

 may, in general, be readily perceived. Its fibres pro- 

 ceed in various directions, according to the kind of 

 motion to be executed, and extend or corrugate the 

 skin at pleasure. 



The appendices of the skin in this class of animals 

 ought to be carefully studied, as they furnish the most 

 obvious marks for distinguishing species, and for con- 

 structing divisions in their systematic arrangement. The 

 appendices of the cuticle are few in number, and perhaps 



